I'm looking for open RPGs to play with my group. By "open-content" I mean games that are licensed similar to open-source software—everyone is allowed to get and modify the game and release the result.

The background is that I don't want to rely too much on a system that might die if its creator drops it for whatever reason. If a game is open-content, I and any community that is interested in it can work on it to keep it alive, at least as long as the campaign runs.

So I would appreciate any listing of such games to pick some from for playing.